(a) Field of the Invention
This invention relates to a ceramic composition, and, more particularly, it is concerned with an alumina ceramic composition having a low dielectric loss in the high frequency region, and being useful as those component parts for the telecommunication apparatuses and appliances such as a dielectric resonator, a substrate for micro-wave integrated circuits, micro-wave transmission window, dielectric members for antenna, and so forth.
(b) Background of the Invention
With development in the telecommunication networks in recent years, practical range of microwave frequency to be used has extended to the high frequency region. In conjunction with this, the dielectric ceramics find their uses in the dielectric resonators, the substrate for the microwave integrated circuits, impedance matching of various kinds of microwave circuits, and so forth to be used in the high frequency region. The dielectric ceramic material of this kind is desired to have its dielectric constant (E) of 10 or so, and its dielectric power factor (than .delta.) of as small a value as possible (for example, tan .delta..ltoreq.7.times.10.sup.-5) and the absolute value of its temperature factor (Tf) for the resonance frequency of as small a value as possible (for example, .vertline.Tf.vertline..ltoreq.30 ppm/.degree.C.). On the other hand, since the alumina ceramic has the dielectric constant (E) of 10 or so and a small value of the dielectric power factor (tan .delta.), and yet the material is available abundantly in its resources, it is highly expected for use as the abovementioned dielectric ceramic material. On the other hand, however, it takes a large absolute value of its temperature factor (Tf) for the resonance frequency of about -60 ppm/.degree.C., the ceramic material has not yet attained its practical usefulness.
In order to solve this problem, there has been proposed a dielectric ceramic composition for the substrate as disclosed in Japanese Laid-Open Patent Application No. 53-149696, which contains therein MgO, TiO.sub.2 and CaO added to alumina at their predetermined ratios. There has also been proposed by the present inventors in Japanese Patent Application No. 59-32113 an alumina ceramic composition composed of Al.sub.2 O.sub.3, CaO and TiO.sub.2 at their given ratios, for the same purpose as mentioned above.
However, these prior patent applications as above referred to are unable to provide the dielectric resonators having sufficient precision in a higher frequency region of 8 GHz or above, hence they are required to be further improved of their dielectric loss in the high frequency region.